This chapter focuses on systems that use halocarbons (halogenated hydrocarbons) as refrigerants. The most commonly used halogen refrigerants are chlorine (Cl) and fluorine (F). Halocarbon refrigerants are classified into four groups: chlorofluorocarbons (CFCs), which contain carbon, chlorine, and fluorine; hydrochlorofluorocarbons (HCFCs), which consist of carbon, hydrogen, chlorine, and fluorine; hydrofluorocarbons (HFCs), which contain carbon, hydrogen, and fluorine; and hydrofluoroolefins (HFOs), which are HFC refrigerants derived from an alkene (olefin; i.e., an unsaturated compound having at least one carbon-to-carbon double bond). Examples of these refrigerants can be found in Chapter 29 of the 2021 ASHRAE Handbook—Fundamentals. Application System Safety Basic Piping Principles Refrigerant Line Sizing Piping at Multiple Compressors Piping at Various System Components Discharge (Hot-Gas) Lines Defrost Gas Supply Lines Heat Exchangers and Vessels Refrigeration Accessories Head Pressure Control for Refrigerant Condensers Keeping Liquid from Crankcase During Off Cycles Hot-Gas Bypass Arrangements Minimizing Refrigerant Charge in Commercial Systems Refrigerant Retrofitting Temperature Glide